fix(a11y): add id=main-content to marketing layout main (#94)

The global skip-link in src/routes/+layout.svelte targets #main-content,
but the marketing layout <main> had no id, so the skip-link did nothing
on marketing routes. The (admin) and (portal) layouts already set this id;
this aligns the marketing layout, satisfying WCAG 2.4.1 (bypass-blocks).

Closes #76

Co-authored-by: dobby-yivi-agent[bot] <275734547+dobby-yivi-agent[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: dobby-coder[bot]

src/routes/(marketing)/+layout.svelte

@@ -11,7 +11,7 @@
 	showRegister={data.marketingFlags.registration}
 	auth={data.auth}
 />
-<main>
+<main id="main-content">
 	{@render children()}
 </main>
 <Footer />